1. Technical Field
The present invention relates to an image decoding device that decodes encoded moving image data, and in particular to decoding technology for reproducing moving image data from halfway through.
2. Background Art
The recent demand for reproduction of an image with high quality on a large screen has increased the data size of an image, especially the data size of a moving image. Assuming, for example, a situation where such a moving image with ever-increasing data size has to be recorded on a recording medium, there have been developed technologies for compressing/encoding such a moving image.
Among highly efficient compressing/encoding technologies is the inter-screen predictive encoding technology that makes use of inter-screen correlations. The inter-screen predictive encoding technology is applied to the international standards of an encoding/compression method.
Examples of the international standards are: the MPEG (Moving Picture Experts Group)-2 standard that is intended to realize television broadcasting with a high image quality, ranging from traditional television broadcasting to high-definition television broadcasting; and the H.264/MPEG-4 AVC (Advanced Video Coding) standard that is intended to realize a high compression rate for mobile/internet use.
Generally, moving image data recorded on a recording medium is often reproduced not only from the start of its bitstream, but also from a mid-point of its bitstream. Also, with regard to a television broadcast, the television broadcast is reproduced from a mid-point of its bitstream, unless the television broadcast is viewed from the very beginning.
Basically, decoding moving image data that has been encoded using the inter-screen predictive encoding method requires use of other image data. This is because such moving image data is encoded in such a manner that only a difference between the moving image data and other image data is recorded, so that the moving image data is compressed with a high compression rate.
For this reason, for example, when the reproducing end receives and attempts to reproduce a bitstream of a television broadcast from halfway through, the reproducing end might not be able to reproduce images of the television broadcast if it has not received data of other images to be referred to (hereafter, “reference images”).
In view of the above, there has been proposed technology for decoding and displaying images of moving image data even when the moving image data is reproduced from halfway through (see Patent Document 1). When there exist no reference images that are required for decoding, this technology performs the decoding by using other substitute images that have been prepared in advance (e.g., grayscale images) as substitutes for the originally required reference images.
Although not a technology intended to reproduce moving image data from halfway through, there has also been proposed technology for generating and displaying images even when image data, which is supposed to be decoded, cannot be read out due to an error or the like and hence cannot be decoded (see Patent Document 2).
Below is a brief explanation of the technology of Patent Document 2. When the image data to be decoded does not exist, this technology regards predicted images as images to be displayed. This technology is described in greater detail later, with reference to FIG. 11.
With use of the above-mentioned technologies, when moving image data cannot be decoded, the moving image data can be reproduced halfway through, although pre-encoded images thereof cannot be displayed.    Patent Document 1:
Japanese Laid-Open Patent Application No. H8-163575    Patent Document 2:
Japanese Laid-Open Patent Application No. H7-111654
However, the correlation between the stated substitute images, which have been prepared in advance as substitutes for the originally required reference images, and the originally required reference images is extremely small. Therefore, use of the substitute images will drastically decrease the reproducibility of the resulting decoded images, i.e., the degree of similarity between pre-encoded images and the resulting decoded images will be extremely low. Likewise, regarding the predicted images as the images to be displayed does not involve utilizing residual data that indicates a difference between the pre-encoded images and the reference images. Accordingly, use of this method will also drastically decrease the reproducibility of the resulting decoded images.
In light of the above, the present invention aims to provide a moving image decoding device that can, in reproducing moving image data from halfway through, generate decoded images with high reproducibility.